Crystal structure of the hemolytic lectin CEL-III isolated from the marine invertebrate Cucumaria echinata: implications of domain structure for its membrane pore-formation mechanism

The Crystal Structure of Bacillus thuringiensis Tpp80Aa1 and Its Interaction with Galactose-Containing Glycolipids

Tpp80Aa1 from Bacillus thuringiensis is a Toxin_10 family protein (Tpp) with reported action against Culex mosquitoes. Here, we demonstrate an expanded target range, showing Tpp80Aa1 is also active against the larvae of Anopheles gambiae and Aedes aegypti mosquitoes. We report the first crystal structure of Tpp80Aa1 at a resolution of 1.8 Å, which shows Tpp80Aa1 consists of two domains: an N-terminal β-trefoil domain resembling a ricin B lectin and a C-terminal putative pore-forming domain sharing structural similarity with the aerolysin family. Similar to other Tpp family members, we observe Tpp80Aa1 binds to the mosquito midgut, specifically the posterior midgut and the gastric caecum. We also identify that Tpp80Aa1 can interact with galactose-containing glycolipids and galactose, and this interaction is critical for exerting full insecticidal action against mosquito target cell lines.

Identification of the Ricin-B-Lectin LdRBLk in the Colorado Potato Beetle and an Analysis of Its Expression in Response to Fungal Infections

Ricin-B-lectins (RBLs) have been identified in many groups of organisms, including coleopterans insects, particularly the Colorado potato beetle Leptinotarsa decemlineata (LdRBLs). We hypothesized that one of these LdRBLs (LdRBLk) may be involved in the immune response to fungal infections. We performed a theoretical analysis of the structure of this protein. Additionally, the expression levels of the LdRBlk gene were measured in L. decemlineata in response to infections with the fungi Metarhizium robertsii and Beauveria bassiana. The expression levels of LdRBlk in the L. decemlineata cuticle and fat body were increased in response to both infections. The induction of LdRBlk expression was dependent on the susceptibility of larvae to the fungi. Upregulation of the LdRBlk gene was also observed in response to other stresses, particularly thermal burns. Elevation of LdRBlk expression was frequently observed to be correlated with the expression of the antimicrobial peptide attacin but was not correlated with hsp90 regulation. Commercially available β-lectin of ricin from Ricinuscommunis was observed to inhibit the germination of conidia of the fungi. We suggest that LdRBLk is involved in antifungal immune responses in the Colorado potato beetle, either exerting fungicidal properties directly or acting as a modulator of the immune response.

The venom proteome of three common scyphozoan jellyfishes (Chrysaora caliparea, Cyanea nozakii and Lychnorhiza malayensis) (Cnidaria: Scyphozoa) from the coastal waters of India

The jellyfish venom stored in nematocysts contains highly toxic compounds comprising of polypeptides, enzymes and other proteins, which form their chemical defence armoury against predators. We have characterized the proteome of crude venom extract from three bloom-forming scyphozoan jellyfish along the south-west coast of India, Chrysaora caliparea, Cyanea nozakii and Lychnorhiza malayensis using a Quadrupole-Time of Flight (Q/TOF) mass spectrometry analysis. The most abundant toxin identified from Chrysaora caliparea and Lychnorhiza malayensis is similar to the pore-forming toxins and metalloproteinases. A protective antioxidant enzyme called peroxiredoxin was found abundantly in Cyanea nozakii. Metalloproteinase identified from the C. caliparea shows similarity with the venom of pit viper (Bothrops pauloensis), while that of L. malayensis was similar to the venom of snakes such as the Bothrops insularis and Bothrops asper. Kininogen-1 is a secreted protein, identified for the first time from the jellyfish L. malayensis. The proteome analysis of Cyanea nozakii, Chrysaora caliparea and Lychnorhiza malayensis contained 20, 12, 8 unique proteins, respectively. Our study characterized the proteome map of crude venom extract from L. malayensis and C. caliparea for the first time, and the venom profile is compared with published information elsewhere. Proteomic data from this study has been made available in the public domain.

Draft genome sequences of Hirudo medicinalis and salivary transcriptome of three closely related medicinal leeches

BACKGROUND

Salivary cell secretion (SCS) plays a critical role in blood feeding by medicinal leeches, making them of use for certain medical purposes even today.

RESULTS

We annotated the Hirudo medicinalis genome and performed RNA-seq on salivary cells isolated from three closely related leech species, H. medicinalis, Hirudo orientalis, and Hirudo verbana. Differential expression analysis verified by proteomics identified salivary cell-specific gene expression, many of which encode previously unknown salivary components. However, the genes encoding known anticoagulants have been found to be expressed not only in salivary cells. The function-related analysis of the unique salivary cell genes enabled an update of the concept of interactions between salivary proteins and components of haemostasis.

CONCLUSIONS

Here we report a genome draft of Hirudo medicinalis and describe identification of novel salivary proteins and new homologs of genes encoding known anticoagulants in transcriptomes of three medicinal leech species. Our data provide new insights in genetics of blood-feeding lifestyle in leeches.

Galactose-Specific, Hemolytic Lectin CEL-III from Cucumaria echinata

CEL-III is a Ca²⁺-dependent and galactose-specific lectin purified from the sea cucumber, Cucumaria echinata; it exhibits hemolytic and hemagglutinating activities. CEL-III consists of the following three distinct domains: two N-terminal carbohydrate-binding domains (1 and 2), which adopt β-trefoil folds such as the B-chain of ricin and are members of the (QXW)₃ motif family, and domain 3, an oligomerization domain. After binding to the cell surface carbohydrate chains through domains 1 and 2, domain 3 self-associates to form transmembrane pores composed of CEL-III heptamers, leading to cell lysis or death. In this chapter, the purification and carbohydrate-coated microplate binding assay of CEL-III are described.

Crystal structures of a β-trefoil lectin from Entamoeba histolytica in monomeric and a novel disulfide bond-mediated dimeric forms

β-Trefoil lectins are galactose/N-acetyl galactosamine specific lectins, which are widely distributed across all kingdoms of life and are known to perform several important functions. However, there is no report available on the characterization of these lectins from protozoans. We have performed structural and biophysical studies on a β-trefoil lectin from Entamoeba histolytica (EntTref), which exists as a mixture of monomers and dimers in solution. Further, we have determined the affinities of EntTref for rhamnose, galactose and different galactose-linked sugars. We obtained the crystal structure of EntTref in a sugar-free form (EntTref_apo) and a rhamnose-bound form (EntTref_rham). A novel Cys residue-mediated dimerization was revealed in the crystal structure of EntTref_apo while the structure of EntTref_rham provided the structural basis for the recognition of rhamnose by a β-trefoil lectin for the first time. To the best of our knowledge, this is the only report of the structural, functional and biophysical characterization of a β-trefoil lectin from a protozoan source and the first report of Cys-mediated dimerization in this class of lectins.

A GM1b/asialo-GM1 oligosaccharide-binding R-type lectin from purplish bifurcate mussels Mytilisepta virgata and its effect on MAP kinases

A 15‐kDa lectin, termed SeviL, was isolated from Mytilisepta virgata (purplish bifurcate mussel). SeviL forms a noncovalent dimer that binds strongly to ganglio‐series GM1b oligosaccharide (Neu5Acɑ2‐3Galβ1‐3GalNAcβ1‐4Galβ1‐4Glc) and its precursor, asialo‐GM1 (Galβ1‐3GalNAcβ1‐4Galβ1‐4Glc). SeviL also interacts weakly with the glycan moiety of SSEA‐4 hexaose (Neu5Acα2‐3Galβ1‐3GalNAcβ1‐3Galα1‐4Galβ1‐4Glc). A partial protein sequence of the lectin was determined by mass spectrometry, and the complete sequence was identified from transcriptomic analysis. SeviL, consisting of 129 amino acids, was classified as an R(icin B)‐type lectin, based on the presence of the QxW motif characteristic of this fold. SeviL mRNA is highly expressed in gills and, in particular, mantle rim tissues. Orthologue sequences were identified in other species of the family Mytilidae, including Mytilus galloprovincialis, from which lectin MytiLec‐1 was isolated and characterized in our previous studies. Thus, mytilid species contain lectins belonging to at least two distinct families (R‐type lectins and mytilectins) that have a common β‐trefoil fold structure but differing glycan‐binding specificities. SeviL displayed notable cytotoxic (apoptotic) effects against various cultured cell lines (human breast, ovarian, and colonic cancer; dog kidney) that possess asialo‐GM1 oligosaccharide at the cell surface. This cytotoxic effect was inhibited by the presence of anti‐asialo‐GM1 oligosaccharide antibodies. With HeLa ovarian cancer cells, SeviL showed dose‐ and time‐dependent activation of kinase MKK3/6, p38 MAPK, and caspase‐3/9. The transduction pathways activated by SeviL via the glycosphingolipid oligosaccharide were triggered apoptosis.

Database

Nucleotide sequence data have been deposited in the GenBank database under accession numbers MK434191, MK434192, MK434193, MK434194, MK434195, MK434196, MK434197, MK434198, MK434199, MK434200, and MK434201.

Novel Ca2+ -independent carbohydrate recognition of the C-type lectins, SPL-1 and SPL-2, from the bivalve Saxidomus purpuratus

Novel Ca2+ -independent C-type lectins, SPL-1 and SPL-2, were purified from the bivalve Saxidomus purpuratus. They are composed of dimers with either identical (SPL-2 composed of two B-chains) or distinct (SPL-1 composed of A- and B-chains) polypeptide chains, and show affinity for N-acetylglucosamine (GlcNAc)- and N-acetylgalactosamine (GalNAc)-containing carbohydrates, but not for glucose or galactose. A database search for sequence similarity suggested that they belong to the C-type lectin family. X-ray crystallographic analysis revealed definite structural similarities between their subunits and the carbohydrate-recognition domain (CRD) of the C-type lectin family. Nevertheless, these lectins (especially SPL-2) showed Ca2+ -independent binding affinity for GlcNAc and GalNAc. The crystal structure of SPL-2/GalNAc complex revealed that bound GalNAc was mainly recognized via its acetamido group through stacking interactions with Tyr and His residues and hydrogen bonds with Asp and Asn residues, while widely known carbohydrate-recognition motifs among the C-type CRD (the QPD [Gln-Pro-Asp] and EPN [Glu-Pro-Asn] sequences) are not involved in the binding of the carbohydrate. Carbohydrate-binding specificities of individual A- and B-chains were examined by glycan array analysis using recombinant lectins produced from Escherichia coli cells, where both subunits preferably bound oligosaccharides having terminal GlcNAc or GalNAc with α-glycosidic linkages with slightly different specificities.

Functional Characterization of OXYL, A SghC1qDC LacNAc-specific Lectin from The Crinoid Feather Star Anneissia Japonica

We identified a lectin (carbohydrate-binding protein) belonging to the complement 1q(C1q) family in the feather star Anneissia japonica (a crinoid pertaining to the phylum Echinodermata). The combination of Edman degradation and bioinformatics sequence analysis characterized the primary structure of this novel lectin, named OXYL, as a secreted 158 amino acid-long globular head (sgh)C1q domain containing (C1qDC) protein. Comparative genomics analyses revealed that OXYL pertains to a family of intronless genes found with several paralogous copies in different crinoid species. Immunohistochemistry assays identified the tissues surrounding coelomic cavities and the arms as the main sites of production of OXYL. Glycan array confirmed that this lectin could quantitatively bind to type-2 N-acetyllactosamine (LacNAc: Galβ1-4GlcNAc), but not to type-1 LacNAc (Galβ1-3GlcNAc). Although OXYL displayed agglutinating activity towards Pseudomonas aeruginosa, it had no effect on bacterial growth. On the other hand, it showed a significant anti-biofilm activity. We provide evidence that OXYL can adhere to the surface of human cancer cell lines BT-474, MCF-7, and T47D, with no cytotoxic effect. In BT-474 cells, OXYL led to a moderate activation of the p38 kinase in the MAPK signaling pathway, without affecting the activity of caspase-3. Bacterial agglutination, anti-biofilm activity, cell adhesion, and p38 activation were all suppressed by co-presence of LacNAc. This is the first report on a type-2 LacNAc-specific lectin characterized by a C1q structural fold.

Molecular and functional characterization of a glycosylated Galactose-Binding lectin from Mytilus californianus

Lectins play crucial roles for innate immune responses in invertebrates by recognizing and eliminating pathogens. In this study, a lectin from the mussel Mytilus californianus (MCL) was identified and characterized. The lectin was purified by affinity chromatography in α-lactose-agarose resin showing an experimental molecular mass of 18000 Da as determined by SDS-PAGE and MALDI-TOF mass spectrometry. It was specific for binding d-galactose and N-Acetyl-d-galactosamine that contained carbohydrate moieties that were also inhibited by melibiose and raffinose. It had the ability to agglutinate all types of human erythrocytes, as well as rabbit red blood cells. Circular dichroism analyzes have indicated that this lectin possessed an α/β fold with a predominance of β structures. This was consistent with the structure of the protein that was determined by the X-ray diffraction techniques. MCL was crystallized in the space group C2₁ and it diffracted to 1.79 Å resolution. Two monomers were found in the asymmetric unit and they formed dimers in solution. The protein has shown to be a member of the β-trefoil family, with three sugar binding sites per monomer. In accord with fluorescence-based thermal shift assays, we observed that the MCL T_m increased about 10 °C in the presence of galactose. Furthermore, we have determined the complete amino acid sequence by cDNA sequencing. The gene had two ORF2 proteins, one resulting in a 180 residue protein with a theoretical molecular mass of 20227 Da, and another resulting in a 150 residue protein with a theoretical molecular mass of 16911 Da. The difference between the theoretical and experimental values was due to the presence of a glycosylation that was observed by the glycosylation assay. A positive microbial agglutination and a growth inhibition activity were observed against Gram-negative and Gram-positive bacteria. The M. californianus lectin is the fourth member of the recently proposed new family of lectins that have been reported to date, occurring only in mollusks belonging to the family Mytilidae. It is the first member to be glycosylated and with a strong tendency to form large oligomers.

Purification, Biochemical Characterization, and Amino Acid Sequence of a Novel Type of Lectin from Aplysia dactylomela Eggs with Antibacterial/Antibiofilm Potential

A new lectin from Aplysia dactylomela eggs (ADEL) was isolated by affinity chromatography on HCl-activated Sepharose™ media. Hemagglutination caused by ADEL was inhibited by several galactosides, mainly galacturonic acid (Ka = 6.05 × 10⁶ M^-1). The primary structure of ADEL consists of 217 residues, including 11 half-cystines involved in five intrachain and one interchain disulfide bond, resulting in a molecular mass of 57,228 ± 2 Da, as determined by matrix-assisted laser desorption/ionization time of flight mass spectrometry. ADEL showed high similarity with lectins isolated from Aplysia eggs, but not with other known lectins, indicating that these lectins could be grouped into a new family of animal lectins. Three glycosylation sites were found in its polypeptide backbone. Data from peptide-N-glycosidase F digestion and MS suggest that all oligosaccharides attached to ADEL are high in mannose. The secondary structure of ADEL is predominantly β-sheet, and its tertiary structure is sensitive to the presence of ligands, as observed by CD. A 3D structure model of ADEL was created and shows two domains connected by a short loop. Domain A is composed of a flat three-stranded and a curved five-stranded β-sheet, while domain B presents a flat three-stranded and a curved four-stranded β-sheet. Molecular docking revealed favorable binding energies for interactions between lectin and galacturonic acid, lactose, galactosamine, and galactose. Moreover, ADEL was able to agglutinate and inhibit biofilm formation of Staphylococcus aureus, suggesting that this lectin may be a potential alternative to conventional use of antimicrobial agents in the treatment of infections caused by Staphylococcal biofilms.

Genome-wide identification and domain organization of lectin domains in cucumber

Lectins are ubiquitous proteins in plants and play important roles in a diverse set of biological processes, such as plant defense and cell signaling. Despite the availability of the Cucumis sativus L. genome sequence since 2009, little is known with respect to the occurrence of lectins in cucumber. In this study, a total of 146 putative lectin genes belonging to 10 different lectin families were identified and localized in the cucumber genome. Domain architecture analysis revealed that most of these lectin gene sequences contain multiple domains, where lectin domains are linked with other domains, as such creating chimeric lectin sequences encoding proteins with dual activities. This study provides an overview of lectin motifs in cucumber and will help to understand their potential biological role(s).

cDNA and Gene Structure of MytiLec-1, A Bacteriostatic R-Type Lectin from the Mediterranean Mussel (Mytilus galloprovincialis)

MytiLec is an α-d-galactose-binding lectin with a unique primary structure isolated from the Mediterranean mussel (Mytilus galloprovincialis). The lectin adopts a β-trefoil fold that is also found in the B-sub-unit of ricin and other ricin-type (R-type) lectins. We are introducing MytiLec(-1) and its two variants (MytiLec-2 and -3), which both possess an additional pore-forming aerolysin-like domain, as members of a novel multi-genic “mytilectin family” in bivalve mollusks. Based on the full length mRNA sequence (911 bps), it was possible to elucidate the coding sequence of MytiLec-1, which displays an extended open reading frame (ORF) at the 5′ end of the sequence, confirmed both at the mRNA and at the genomic DNA sequence level. While this extension could potentially produce a polypeptide significantly longer than previously reported, this has not been confirmed yet at the protein level. MytiLec-1 was revealed to be encoded by a gene consisting of two exons and a single intron. The first exon comprised the 5′UTR and the initial ATG codon and it was possible to detect a putative promoter region immediately ahead of the transcription start site in the MytiLec-1 genomic locus. The remaining part of the MytiLec-1 coding sequence (including the three sub-domains, the 3′UTR and the poly-A signal) was included in the second exon. The bacteriostatic activity of MytiLec-1 was determined by the agglutination of both Gram-positive and Gram-negative bacteria, which was reversed by the co-presence of α-galactoside. Altogether, these data support the classification of MytiLec-1 as a member of the novel mytilectin family and suggest that this lectin may play an important role as a pattern recognition receptor in the innate immunity of mussels.

Effects of amino acid mutations in the pore-forming domain of the hemolytic lectin CEL-III

The hemolytic lectin CEL-III forms transmembrane pores in the membranes of target cells. A study on the effect of site-directed mutation at Lys405 in domain 3 of CEL-III indicated that replacements of this residue by relatively smaller residues lead to a marked increase in hemolytic activity, suggesting that moderately destabilizing domain 3 facilitates formation of transmembrane pores through conformational changes.

Novel hemagglutinating, hemolytic and cytotoxic activities of the intermediate subunit of Entamoeba histolytica lectin

Galactose and N-acetyl-D-galactosamine (Gal/GalNAc) inhibitable lectin of Entamoeba histolytica, a common protozoan parasite, has roles in pathogenicity and induction of protective immunity in mouse models of amoebiasis. The lectin consists of heavy (Hgl), light (Lgl), and intermediate (Igl) subunits. Hgl has lectin activity and Lgl does not, but little is known about the activity of Igl. In this study, we assessed various regions of Igl for hemagglutinating activity using recombinant proteins expressed in Escherichia coli. We identified a weak hemagglutinating activity of the protein. Furthermore, we found novel hemolytic and cytotoxic activities of the lectin, which resided in the carboxy-terminal region of the protein. Antibodies against Igl inhibited the hemolytic activity of Entamoeba histolytica trophozoites. This is the first report showing hemagglutinating, hemolytic and cytotoxic activities of an amoebic molecule, Igl.

A review of the immune molecules in the sea cucumber

It is very important to identify and characterize the immune-related genes that respond to pathogens. Until recently, only some of the immune-related genes in sea cucumbers had been characterized. Their expression patterns after pathogen challenges have been analyzed via expressed sequence tag libraries, microarray studies and proteomic approaches. These genes include lectins, antimicrobial peptides, lysozyme, enzymes, clotting protein, pattern recognition proteins, Toll receptors, complement C3 and other humoral factors that might participate in the innate immune system of sea cucumbers. Although the participation of some of these immune molecules in the sea cucumber's innate immune defense against invading pathogens has been demonstrated, the functions of many of the molecules remain unclear. This review focuses on the discovery and functional characterization of the immune-related molecules from the sea cucumber for the first time and provides new insights into the immune mechanisms of the sea cucumber, which opens new possibilities for developing drugs for novel anti-bacterial and antiviral applications in fisheries.

Hemolytic lectin CEL-III heptamerizes via a large structural transition from α-helices to a β-barrel during the transmembrane pore formation process

CEL-III is a hemolytic lectin isolated from the sea cucumber Cucumaria echinata. This lectin is composed of two carbohydrate-binding domains (domains 1 and 2) and one oligomerization domain (domain 3). After binding to the cell surface carbohydrate chains through domains 1 and 2, domain 3 self-associates to form transmembrane pores, leading to cell lysis or death, which resembles other pore-forming toxins of diverse organisms. To elucidate the pore formation mechanism of CEL-III, the crystal structure of the CEL-III oligomer was determined. The CEL-III oligomer has a heptameric structure with a long β-barrel as a transmembrane pore. This β-barrel is composed of 14 β-strands resulting from a large structural transition of α-helices accommodated in the interface between domains 1 and 2 and domain 3 in the monomeric structure, suggesting that the dissociation of these α-helices triggered their structural transition into a β-barrel. After heptamerization, domains 1 and 2 form a flat ring, in which all carbohydrate-binding sites remain bound to cell surface carbohydrate chains, stabilizing the transmembrane β-barrel in a position perpendicular to the plane of the lipid bilayer.

Identification of the amino acid residues involved in the hemolytic activity of the Cucumaria echinata lectin CEL-III

BACKGROUND

CEL-III is a hemolytic lectin isolated from the sea cucumber Cucumaria echinata that shows Ca(2+)-dependent Gal/GalNAc-binding specificity. This lectin is composed of two carbohydrate-recognition domains (domains 1 and 2) and an oligomerization domain (domain 3) that facilitates CEL-III assembly in the target cell membrane to form ion-permeable pores.

METHODS

Several amino acid residues in domain 3 were replaced by alanine, and hemolytic activity of the mutants was examined.

RESULTS

K344A, K351A, K405A, K420A and K425A showed marked increases in activity. In particular, K405A had activity that was 360-fold higher than the wild-type recombinant CEL-III and 3.6-fold higher than the native protein purified from sea cucumber. Since these residues appear to play roles in the stabilization of domain 3 through ionic and hydrogen bonding interactions with other residues, the mutations of these residues presumably lead to destabilization of domain 3, which consequently induces the oligomerization of the protein through association of domain 3 in the membrane. In contrast, K338A, R378A and R408A mutants exhibited a marked decrease in hemolytic activity. Since these residues are located on the surface of domain 3 without significant interactions with other residue, they may be involved in the interaction with components of the target cell membrane.

CONCLUSIONS

Several amino acid residues, especially basic residues, are found to be involved in the hemolytic activity as well as the oligomerization ability of CEL-III.

GENERAL SIGNIFICANCE

The results provide important clues to the membrane pore-forming mechanism of CEL-III, which is also related to that of bacterial pore-forming toxins.

Crystallization and preliminary crystallographic study of oligomers of the haemolytic lectin CEL-III from the sea cucumber Cucumaria echinata

CEL-III is a Ca(2+)-dependent haemolytic lectin isolated from the marine invertebrate Cucumaria echinata. This lectin binds to Gal/GalNAc-containing carbohydrate chains on the cell surface and, after conformational changes, oligomerizes to form ion-permeable pores in cell membranes. CEL-III also forms soluble oligomers similar to those formed in cell membranes upon binding of specific carbohydrates in high-pH and high-salt solutions. These soluble and membrane CEL-III oligomers were crystallized and X-ray diffraction data were collected. Crystals of soluble oligomers and membrane oligomers diffracted X-rays to 3.3 and 4.2 Å resolution, respectively, using synchrotron radiation and the former was found to belong to space group C2. Self-rotation functional analysis of the soluble oligomer crystal suggested that it might be composed of heptameric CEL-III.

Expression of putative immune response genes during early ontogeny in the coral Acropora millepora

BACKGROUND

Corals, like many other marine invertebrates, lack a mature allorecognition system in early life history stages. Indeed, in early ontogeny, when corals acquire and establish associations with various surface microbiota and dinoflagellate endosymbionts, they do not efficiently distinguish between closely and distantly related individuals from the same population. However, very little is known about the molecular components that underpin allorecognition and immunity responses or how they change through early ontogeny in corals.

METHODOLOGY/PRINCIPAL FINDINGS

Patterns in the expression of four putative immune response genes (apextrin, complement C3, and two CELIII type lectin genes) were examined in juvenile colonies of Acropora millepora throughout a six-month post-settlement period using quantitative real-time PCR (qPCR). Expression of a CELIII type lectin gene peaked in the fourth month for most of the coral juveniles sampled and was significantly higher at this time than at any other sampling time during the six months following settlement. The timing of this increase in expression levels of putative immune response genes may be linked to allorecognition maturation which occurs around this time in A. millepora. Alternatively, the increase may represent a response to immune challenges, such as would be involved in the recognition of symbionts (such as Symbiodinium spp. or bacteria) during winnowing processes as symbioses are fine-tuned.

CONCLUSIONS/SIGNIFICANCE

Our data, although preliminary, are consistent with the hypothesis that lectins may play an important role in the maturation of allorecognition responses in corals. The co-expression of lectins with apextrin during development of coral juveniles also raises the possibility that these proteins, which are components of innate immunity in other invertebrates, may influence the innate immune systems of corals through a common pathway or system. However, further studies investigating the expression of these genes in alloimmune-challenged corals are needed to further clarify emerging evidence of a complex innate immunity system in corals.

Purification and characterization of hemagglutinating proteins from Poker-chip Venus (Meretrix lusoria) and Corbicula clam (Corbicula fluminea)

Hemagglutinating proteins (HAPs) were purified from Poker-chip Venus (Meretrix lusoria) and Corbicula clam (Corbicula fluminea) using gel-filtration chromatography on a Sephacryl S-300 column. The molecular weights of the HAPs obtained from Poker-chip Venus and Corbicula clam were 358 kDa and 380 kDa, respectively. Purified HAP from Poker-chip Venus yielded two subunits with molecular weights of 26 kDa and 29 kDa. However, only one HAP subunit was purified from Corbicula clam, and its molecular weight was 32 kDa. The two Poker-chip Venus HAPs possessed hemagglutinating ability (HAA) for erythrocytes of some vertebrate animal species, especially tilapia. Moreover, HAA of the HAP purified from Poker-chip Venus was higher than that of the HAP of Corbicula clam. Furthermore, Poker-chip Venus HAPs possessed better HAA at a pH higher than 7.0. When the temperature was at 4°C–10°C or the salinity was less than 0.5‰, the two Poker-chip Venus HAPs possessed better HAA compared with that of Corbicula clam.

Identification of fast-evolving genes in the scleractinian coral Acropora using comparative EST analysis

To identify fast-evolving genes in reef-building corals, we performed direct comparative sequence analysis with expressed sequence tag (EST) datasets from two acroporid species: Acropora palmata from the Caribbean Sea and A. millepora from the Great Barrier Reef in Australia. Comparison of 589 independent sequences from 1,421 A. palmata contigs, with 10,247 A. millepora contigs resulted in the identification of 196 putative homologues. Most of the homologous pairs demonstrated high amino acid similarities (over 90%). Comparisons of putative homologues showing low amino acid similarities (under 90%) among the Acropora species to the near complete datasets from two other cnidarians (Hydra magnipapillata and Nematostella vectensis) implied that some were non-orthologous. Within 86 homologous pairs, 39 exhibited dN/dS ratios significantly less than 1, suggesting that these genes are under purifying selection associated with functional constraints. Eight independent genes showed dN/dS ratios exceeding 1, while three deviated significantly from 1, suggesting that these genes may play important roles in the adaptive evolution of Acropora. Our results also indicated that CEL-III lectin was under positive selection, consistent with a possible role in immunity or symbiont recognition. Further studies are needed to clarify the possible functions of the genes under positive selection to provide insight into the evolutionary process of corals.

High-resolution structural insights on the sugar-recognition and fusion tag properties of a versatile β-trefoil lectin domain from the mushroom Laetiporus sulphureus

In this work, we analyzed at high resolution the sugar-binding mode of the recombinant N-terminal ricin-B domain of the hemolytic protein LSLa (LSL(150)) from the mushroom Laetiporus sulphureus and also provide functional in vitro evidence suggesting that, together with its putative receptor-binding role, this module may also increase the solubility of its membrane pore-forming partner. We first demonstrate that recombinant LSL(150) behaves as an autonomous folding unit and an active lectin. We have determined its crystal structure at 1.47 Å resolution and also that of the [LSL(150):(lactose)β, γ)] binary complex at 1.67 Å resolution. This complex reveals two lactose molecules bound to the β and γ sites of LSL(150), respectively. Isothermal titration calorimetry indicates that LSL(150) binds two lactoses in solution with highly different affinities. Also, we test the working hypothesis that LSL(150) exhibits in vivo properties typical of solubility tags. With this aim, we have fused an engineered version of LSL(150) (LSL(t)) to the N-terminal end of various recombinant proteins. All the designed LSL(150)-tagged fusion proteins were successfully produced at high yield, and furthermore, the target proteins were purified by a straightforward affinity procedure on agarose-based matrices due to the excellent properties of LSL(150) as an affinity tag. An optimized protocol for target protein purification was devised, which involved removal of the LSL(150) tag through in-column cleavage of the fusion proteins with His(6)-tagged TEV endoprotease. These results permitted to set up a novel, lectin-based system for production and purification of recombinant proteins in E. coli cells with attractive biotechnological applications.

A lytic mechanism based on soluble phospholypases A2 (sPLA2) and β-galactoside specific lectins is exerted by Ciona intestinalis (ascidian) unilocular refractile hemocytes against K562 cell line and mammalian erythrocytes

Hemocytes from the ascidian Ciona intestinalis exert in vitro Ca²+-dependent cytotoxic activity toward mammalian erythrocytes and K562 cells. To examine the lytic mechanism, hemocyte populations were separated (B1-B6 bands) through a Percoll discontinuous density gradient, the hemocyte cytotoxic activity (HCA) and the lytic activity of the hemocyte lysate supernatant (HLS) were assayed. In addition the separated hemocytes were cultured and the cell-free culture medium (CFM) assayed after 3 h culture. Results support that unilocular refractile hemocytes (URGs), enriched in B5, are cytotoxic. The B5-HLS contains lysins and the activity of B5-CFM shows that lysins can be released into a culture medium. The B5 activity was blocked by D-galactose, α-lactose, lactulose, LacNAc, thiodigalactoside (TDG), L-fucose, D-mannose, D-glucose, sphingomyelin (SM), and soluble phospholipase A2 (sPLA2) inhibitors (dibucain, quinacrine). Accordingly, HLS chemico-physical properties (alkaline medium, high thermostability, Ca²+-dependence, trypsin treatment, protease inhibitors) and SEM observations of the affected targets suggested that sPLA2 could be responsible for changes and large alterations of the target cell membrane. An apoptotic activity, as recorded by a caspase 3, 7 assay, was found by treating K562 cells with very diluted HLS. A lytic mechanism involving sPLA2 and lectins promptly released by URGs and morula cells respectively is suggested, whereas target cell membrane SM could be a modulator of the enzyme activity.

Galactose recognition by a tetrameric C-type lectin, CEL-IV, containing the EPN carbohydrate recognition motif

CEL-IV is a C-type lectin isolated from a sea cucumber, Cucumaria echinata. This lectin is composed of four identical C-type carbohydrate-recognition domains (CRDs). X-ray crystallographic analysis of CEL-IV revealed that its tetrameric structure was stabilized by multiple interchain disulfide bonds among the subunits. Although CEL-IV has the EPN motif in its carbohydrate-binding sites, which is known to be characteristic of mannose binding C-type CRDs, it showed preferential binding of galactose and N-acetylgalactosamine. Structural analyses of CEL-IV-melibiose and CEL-IV-raffinose complexes revealed that their galactose residues were recognized in an inverted orientation compared with mannose binding C-type CRDs containing the EPN motif, by the aid of a stacking interaction with the side chain of Trp-79. Changes in the environment of Trp-79 induced by binding to galactose were detected by changes in the intrinsic fluorescence and UV absorption spectra of WT CEL-IV and its site-directed mutants. The binding specificity of CEL-IV toward complex oligosaccharides was analyzed by frontal affinity chromatography using various pyridylamino sugars, and the results indicate preferential binding to oligosaccharides containing Galβ1-3/4(Fucα1-3/4)GlcNAc structures. These findings suggest that the specificity for oligosaccharides may be largely affected by interactions with amino acid residues in the binding site other than those determining the monosaccharide specificity.

Laetiporus sulphureus lectin and aerolysin protein family

The parasitic mushroom Laetiporus sulphureus produces a family of lectins (LSL's) sharing 80-90% sequence identity that possesses a low but significant sequence similarity to the bacterial pore-forming toxins mosquitocidal toxin Mtx-2 from Bacillus sphaericus and a toxin from Clostridium septicum. The crystal structure of one member of the L. sulphureus lectins family (LSLa) reveals unexpected structural similarities to the 1-pore-forming toxins from the aerolysin family, namely, aerolysin from the Gram-negative bacterium Aeromonas hydrophila, epsilon-toxin from Clostridium perfringens and parasporin from B. thuringiensis. This similarity presumably indicates that the hemolytic activity of LSLa proceeds through a molecular mechanism that involves the formation of oligomeric transmembrane beta-barrels. Comparison of the crystal structures of the above mentioned proteins reveals common pore-forming modules, which are then distributed both in bacteria and fungi. Currently, it can be stated that the above three dimensional structures have been key in revealing structural similarities that were elusive at the sequence level. A potential corollary from this is that structural studies aimed at determining high resolution structures of aerolysin-like pore-forming toxins, whose biological activity involves large conformational changes, are mandatory to define protein domains or structural motifs with membrane-binding properties.

New tricks with old genes: the genetic bases of novel cnidarian traits

Recent thought on genome evolution has focused on the creation of new genes and changes in regulatory mechanisms while ignoring the role of selective gene loss in shaping genomes. Using data from two cnidarians, the jellyfish Clytia and the coral Acropora, we examined the relative significance of new 'taxonomically restricted' genes and selectively retained ancestral genes in enabling the evolution of novel traits. Consistent with its more complex life-cycle, the proportion of novel genes identified in Clytia was higher than that in the 'polyp only' cnidarians Nematostella and Hydra, but each of these cnidarians has retained a proportion of ancestral genes not present in the other two. The ubiquity and near-stochastic nature of gene loss can explain the discord between patterns of gene distribution and taxonomy.

NMR studies on the interaction of sugars with the C-terminal domain of an R-type lectin from the earthworm Lumbricus terrestris

The R-type lectin EW29, isolated from the earthworm Lumbricus terrestris, consists of two homologous domains (14,500 Da) showing 27% identity with each other. The C-terminal domain (Ch; C-half) of EW29 (EW29Ch) has two sugar-binding sites in subdomains alpha and gamma, and the protein uses these sugar-binding sites for its function as a single-domain-type hemagglutinin. In order to determine the sugar-binding ability and specificity for each of the two sugar-binding sites in EW29Ch, ligand-induced chemical-shift changes in EW29Ch were monitored using (1)H-(15)N HSQC spectra as a function of increasing concentrations of lactose, melibiose, D-galactose, methyl alpha-D-galactopyranoside and methyl beta-D-galactopyranoside. Shift perturbation patterns for well-resolved resonances confirmed that all of these sugars associated independently with the two sugar-binding sites of EW29Ch. NMR titration experiments showed that the sugar-binding site in subdomain alpha had a slow or intermediate exchange regime on the chemical-shift timescale (K(d) = 10(-2) to 10(-1) mM), whereas that in subdomain gamma had a fast exchange regime for these sugars (K(d) = 2-6 mM). Thus, our results suggest that the two sugar-binding sites of EW29Ch in the same molecule retain its hemagglutinating activity, but this activity is 10-fold lower than that of the whole protein because EW29Ch has two sugar-binding sites in the same molecule, one of which has a weak binding mode.

Hemolytic C-type lectin CEL-III from sea cucumber expressed in transgenic mosquitoes impairs malaria parasite development

The midgut environment of anopheline mosquitoes plays an important role in the development of the malaria parasite. Using genetic manipulation of anopheline mosquitoes to change the environment in the mosquito midgut may inhibit development of the malaria parasite, thus blocking malaria transmission. Here we generate transgenic Anopheles stephensi mosquitoes that express the C-type lectin CEL-III from the sea cucumber, Cucumaria echinata, in a midgut-specific manner. CEL-III has strong and rapid hemolytic activity toward human and rat erythrocytes in the presence of serum. Importantly, CEL-III binds to ookinetes, leading to strong inhibition of ookinete formation in vitro with an IC₅₀ of 15 nM. Thus, CEL-III exhibits not only hemolytic activity but also cytotoxicity toward ookinetes. In these transgenic mosquitoes, sporogonic development of Plasmodium berghei is severely impaired. Moderate, but significant inhibition was found against Plasmodium falciparum. To our knowledge, this is the first demonstration of stably engineered anophelines that affect the Plasmodium transmission dynamics of human malaria. Although our laboratory-based research does not have immediate applications to block natural malaria transmission, these findings have significant implications for the generation of refractory mosquitoes to all species of human Plasmodium and elucidation of mosquito–parasite interactions.

Malaria is arguably the most important vector-borne disease worldwide, affecting 300 million people and killing 1–2 million people every year. The lack of an effective vaccine and the emergence of the parasites' resistance to many existing anti-malarial drugs have aggravated the situation. Clearly, development of novel strategies for control of the disease is urgently needed. Mosquitoes are obligatory vectors for the disease and inhibition of parasite development in the mosquito has considerable promise as a new approach in the fight against malaria. Based on recent advances in the genetic engineering of mosquitoes, the concept of generating genetically modified (GM) mosquitoes that hinder transmission by either killing or interfering with parasite development is a potential means of controlling the disease. To generate these GM mosquitoes, the authors focused on a unique lectin isolated from the sea cucumber, which has both hemolytic and cytotoxic activities, as an anti-parasite effector molecule. A transgenic mosquito expressing the lectin effectively caused erythrocyte lysis in the midgut after ingestion of an infectious blood meal and severely impaired parasite development. This laboratory-acquired finding may provide significant implications for future malaria control using GM mosquitoes refractory to the parasites.

Isolation, purification, characterization and glycan-binding profile of a d-galactoside specific lectin from the marine sponge, Halichondria okadai

A lectin recognizing both Galbeta1-3GlcNAc and Galbeta1-4GlcNAc was purified from the demosponge Halichondria okadai by lactosyl-agarose affinity chromatography. The molecular mass of the lectin was determined to be 30 kDa by SDS-PAGE under reducing and non-reducing conditions and 60 kDa by gel permeation chromatography. The pI value of the lectin was 6.7. It was found to agglutinate trypsinized and glutaraldehyde-fixed rabbit and human erythrocytes in the presence and absence of divalent cations. The hemagglutinating activity by the lectin was inhibited by d-galactose, methyl-d-galactopyranoside, N-acetyl-d-galactosamine, methyl-N-acetyl-d-galactosaminide, lactose, melibiose, and asialofetuin. The K(d) of the lectin against p-nitrophenyl-beta-lactoside was determined to be 2.76x10(-5) M and its glycan-binding profile given by frontal affinity chromatography was shown to be similar to many other known galectins. Partial primary structure analysis of 7 peptides by cleavage with lysyl endopeptidase indicated that one of the peptides showed significant similarity with galectin purified from the sponge Geodia cydonium.

Marine pharmacology in 2003-4: marine compounds with anthelmintic antibacterial, anticoagulant, antifungal, anti-inflammatory, antimalarial, antiplatelet, antiprotozoal, antituberculosis, and antiviral activities; affecting the cardiovascular, immune and nervous systems, and other miscellaneous mechanisms of action

The current marine pharmacology review that covers the peer-reviewed literature during 2003 and 2004 is a sequel to the authors' 1998-2002 reviews, and highlights the preclinical pharmacology of 166 marine chemicals derived from a diverse group of marine animals, algae, fungi and bacteria. Anthelmintic, antibacterial, anticoagulant, antifungal, antimalarial, antiplatelet, antiprotozoal, antituberculosis or antiviral activities were reported for 67 marine chemicals. Additionally 45 marine compounds were shown to have significant effects on the cardiovascular, immune and nervous system as well as possessing anti-inflammatory effects. Finally, 54 marine compounds were reported to act on a variety of molecular targets and thus may potentially contribute to several pharmacological classes. Thus, during 2003-2004, research on the pharmacology of marine natural products which involved investigators from Argentina, Australia, Brazil, Belgium, Canada, China, France, Germany, India, Indonesia, Israel, Italy, Japan, Mexico, Morocco, the Netherlands, New Zealand, Norway, Panama, the Philippines, Portugal, Russia, Slovenia, South Korea, Spain, Thailand, Turkey, United Kingdom, and the United States, contributed numerous chemical leads for the continued global search for novel therapeutic agents with broad spectrum activity.

Transcriptome analysis of expressed sequence tags from the venom glands of the fish Thalassophryne nattereri

Thalassophryne nattereri (niquim) is a venomous fish found on the northern and northeastern coasts of Brazil. Every year, hundreds of humans are affected by the poison, which causes excruciating local pain, edema, and necrosis, and can lead to permanent disabilities. In experimental models, T. nattereri venom induces edema and nociception, which are correlated to human symptoms and dependent on venom kininogenase activity; myotoxicity; impairment of blood flow; platelet lysis and cytotoxicity on endothelial cells. These effects were observed with minute amounts of venom. To characterize the primary structure of T. nattereri venom toxins, a list of transcripts within the venom gland was made using the expressed sequence tag (EST) strategy. Here we report the analysis of 775 ESTs that were obtained from a directional cDNA library of T. nattereri venom gland. Of these ESTs, 527 (68%) were related to sequences previously described. These were categorized into 10 groups according to their biological functions. Sequences involved in gene and protein expression accounted for 14.3% of the ESTs, reflecting the important role of protein synthesis in this gland. Other groups included proteins engaged in the assembly of disulfide bonds (0.5%), chaperones involved in the folding of nascent proteins (1.4%), and sequences related to clusterin (1.5%), as well as transcripts related to calcium binding proteins (1.0%). We detected a large cluster (1.3%) related to cocaine- and amphetamine-regulated transcript (CART), a peptide involved in the regulation of food intake. Surprisingly, several retrotransposon-like sequences (1.0%) were found in the library. It may be that their presence accounts for some of the variation in venom toxins. The toxin category (18.8%) included natterins (18%), which are a new group of kininogenases recently described by our group, and a group of C-type lectins (0.8%). In addition, a considerable number of sequences (32%) was not related to sequences in the databases, which indicates that a great number of new toxins and proteins are still to be discovered from this fish venom gland.

Characterization of a recombinant C-type lectin, rCEL-IV, expressed in Escherichia coli cells using a synthetic gene

The body fluid of marine invertebrate Cucumaria echinata (Holothuroidea) contains four Ca2+-dependent galactose-specific lectins. One of these lectins, CEL-IV, is composed of a C-type carbohydrate-recognition domain homotetramer. CEL-IV exhibits higher specificity for alpha-galactosides than for beta-galactosides, while other C. echinata lectins show preferential binding of beta-galactosides. We constructed an artificial synthetic gene for recombinant CEL-IV (rCEL-IV) based on the amino acid sequence previously determined from the purified protein. rCEL-IV was expressed in Escherichia coli cells as inclusion bodies. After the refolding process, most of rCEL-IV spontaneously formed a homotetramer structure having interchain disulfide bonds. The secondary structure of rCEL-IV was similar to that of the native one, as judged by the comparison of the far UV-circular dichroism spectra of rCEL-IV and native CEL-IV (nCEL-IV). Carbohydrate-binding specificity of rCEL-IV was confirmed to be similar to that of nCEL-IV from the results of the binding-inhibition assay using liposomes composed of rabbit erythrocyte lipids. Crystals of rCEL-IV were obtained in a few days by the sitting drop vapor diffusion method. These results indicate that rCEL-IV achieved essentially correct three-dimensional structure, including the carbohydrate-binding sites, and it would be very useful for further study on the carbohydrate-recognition mechanism by mutational and X-ray crystallographic analyses.

A beta-galactose-specific lectin isolated from the marine worm Chaetopterus variopedatus possesses anti-HIV-1 activity

A 30 kDa beta-galactose-specific lectin named CVL was isolated from the polychaete marine worm Chaetopterus variopedatus (Annelida) and its anti-HIV-1 activity in vitro was determined. Results showed that CVL inhibited cytopathic effect induced by HIV-1 and the production of viral p24 antigen. The EC(50) values were 0.0043 and 0.057 microM, respectively. Time-of-addition analysis of anti-HIV-1 activity indicated its action was at the early stage of virus replication. CVL could blocked the cell-to-cell fusion process of HIV infected and uninfected cells with an EC(50) of 0.073 microM. The inhibition of HIV-1 entry into host cells was demonstrated by using fluorescence-based real-time quantify PCR. At CVL concentration of 0.33 microM and 0.07 microM, 86% and 21% virus attachment were blocked, respectively. The anti-HIV-1 action of CVL might relate to blockade of HIV-1 entry into cells.

Structural Analysis of the Laetiporus sulphureus Hemolytic Pore-forming Lectin in Complex with Sugars

LSL is a lectin produced by the parasitic mushroom Laetiporus sulphureus, which exhibits hemolytic and hemagglutinating activities. Here, we report the crystal structure of LSL refined to 2.6-A resolution determined by the single isomorphous replacement method with the anomalous scatter (SIRAS) signal of a platinum derivative. The structure reveals that LSL is hexameric, which was also shown by analytical ultracentrifugation. The monomeric protein (35 kDa) consists of two distinct modules: an N-terminal lectin module and a pore-forming module. The lectin module has a beta-trefoil scaffold that bears structural similarities to those present in toxins known to interact with galactose-related carbohydrates such as the hemagglutinin component (HA1) of the progenitor toxin from Clostridium botulinum, abrin, and ricin. On the other hand, the C-terminal pore-forming module (composed of domains 2 and 3) exhibits three-dimensional structural resemblances with domains 3 and 4 of the beta-pore-forming toxin aerolysin from the Gram-negative bacterium Aeromonas hydrophila, and domains 2 and 3 from the epsilon-toxin from Clostridium perfringens. This finding reveals the existence of common structural elements within the aerolysin-like family of toxins that could be directly involved in membrane-pore formation. The crystal structures of the complexes of LSL with lactose and N-acetyllactosamine reveal two dissacharide-binding sites per subunit and permits the identification of critical residues involved in sugar binding.

Characteristic Recognition of N-Acetylgalactosamine by an Invertebrate C-type Lectin, CEL-I, Revealed by X-ray Crystallographic Analysis

CEL-I is a C-type lectin, purified from the sea cucumber Cucumaria echinata, that shows a high specificity for N-acetylgalactosamine (GalNAc). We determined the crystal structures of CEL-I and its complex with GalNAc at 2.0 and 1.7 A resolution, respectively. CEL-I forms a disulfide-linked homodimer and contains two intramolecular disulfide bonds, although it lacks one intramolecular disulfide bond that is widely conserved among various C-type carbohydrate recognition domains (CRDs). Although the sequence similarity of CEL-I with other C-type CRDs is low, the overall folding of CEL-I was quite similar to those of other C-type CRDs. The structure of the complex with GalNAc revealed that the basic recognition mode of GalNAc was very similar to that for the GalNAc-binding mutant of the mannose-binding protein. However, the acetamido group of GalNAc appeared to be recognized more strongly by the combination of hydrogen bonds to Arg115 and van der Waals interaction with Gln70. Mutational analyses, in which Gln70 and/or Arg115 were replaced by alanine, confirmed that these residues contributed to GalNAc recognition in a cooperative manner.